There is extensive prior art literature describing methods for selective oxidation of olefins to alpha, beta-unsaturated aldehydes and carboxylic acids. An important aspect of selective oxidation studies have involved conversion of isobutylene to methacrolein and methacrylic acid. The yields of methacrolein obtained with known catalyst systems have generally been lower than the yields achieved in the selective oxidation of propylene to acrolein.
There is continuing development effort in progress on new catalyst systems for improving yield and efficiency of isobutylene oxidation to methacrolein and methacrylic acid.
Of primary importance, is the achievement of the highest possible single-pass yield of useful products, because it is generally uneconomical to recover and recycle unreacted isobutylene. However, as the conversion rate of isobutylene is increased, the quantity of undesirable products produced, such as carbon dioxide, carbon monoxide and acetic acid also increases because of oxidative decomposition. This causes a decrease in the selectivity to methacrolein and methacrylic acid. In addition to the requirement for high single-pass yields of product, the catalyst system must also have a sufficient activity as well as possess a sufficient lifetime so as to be useable in industrial applications.
U.S. Pat. No. 3,879,453 describes an improved process for producing methacrolein and methacrylic acid by contacting isobutylene with molecular oxygen in vapor phase in the presence of a new catalyst system. The catalyst consists of a mixture of solid metal oxides corresponding to the empirical formula Sb.sub.a V.sub.b Mo.sub.c Te.sub.d W.sub.e M.sub.f O.sub.g. wherein M represents at least one element selected from the group consisting of bismuth, thallium and uranium, and wherein the atomic ratio of elements in the formula is such that a=10, b=0.1-10, c=0.4-15, d=0.1-10, e=0-2 and g=16-127.
U.S. Pat. No. 3,939,208 discloses an improved process for producing methacrolein which involves contacting isobutylene with oxygen or oxygen-containing gaseous mixtures in vapor phase in the presence of a new catalyst composition corresponding to the formula Me.sub.v Te.sub.x Ce.sub.y Mo.sub.12 O.sub.z, wherein Me is Na, K, Li, Rb and/or Cs, v=0.3-18, x=0.3-24, y=0.3-21 and z corresponds to the quantity of oxygen bound to the other elements.
There remains a need for improved selective oxidation systems adapted for the conversion of isobutylene to methacrolein and methacrylic acid with high single pass conversion and efficiency.
Accordingly, it is an object of this invention to provide a method for preparing an improved selective oxidation catalyst.
It is another object of this invention to provide a novel molybdate catalyst adapted for selective oxidation of olefins to alpha, beta-unsaturated aldehydes and carboxylic acids.
It is a further object of this invention to provide an improved process for selectively oxidizing isobutylene to methacrolein and methacrylic acid in high single-pass yield.
Other objects and advantages shall become apparent from the accompanying description and examples.